The present invention generally relates to a method for fabricating micro-lens for color filters and devices fabricated and more particularly, relates to a method for fabricating long focal length micro-lens for color filters and devices fabricated by such method.
Color fillers have been used in image sensors for CCD or CMOS applications for producing color images. An essential part of the color filters is the multiplicity of micro-lenses that is formed on top of the color filters in order to focus a light beam. Micro-lenses can be fabricated by a variety of different methods. one of the more frequently used methods is by a standard photolithographic technique followed by reflowing photoresist strips that are formed in cylinders or squares into hemispheres. The technique is carried out by exposing the photoresist to process temperatures in excess of the glass transition temperature of the micro-lens material. At a temperature higher than the glass transition temperature, the cohesive force in the material that forms the solid patterns causes the surface area to be minimized forming a hemisphere.
A typical process for forming a multiplicity of micro-lenses for color filters is shown in FIGS. 1A and 1B. Referring now to FIG. 1A, wherein a process flow chart 10 for fabricating a micro-lens is shown. The process flow-chart 10 corresponds to the structure 20 shown in FIG. 1B for a CMOS image sensor including a multiplicity of micro-lenses 12 and a multiplicity of color filters 22. As shown in FIG. 1B, a photodiode 24 is first formed in the surface of a substrate 26, which also includes a series of metal conductors 28 covered by a layer of passivation 30 that forms an irregular upper surface 32. The irregular upper surface 32 is then planarized by depositing a layer of dielectric material 34 over the passivation layer 30. As shown in FIG. 1A, after the planarization step 14 has been performed, a subsequent step 16 is carried out to form a color filter layer 16 providing red, green and blue color elements. As a final step, a micro-lens spacer 36 is applied by step 18 following which a micro-lens 12 is formed by step 38.
A more detailed view of the conventional color filter 20 is shown in FIGS. 2A, 2B, 3A and 3B. FIG. 2A illustrates an enlarged, cross-sectional view of the image sensor 20 before the micro-lens reflow process, while FIG. 2B illustrates an enlarged, cross-sectional view of the image sensor 20 after the micro-lens reflow process. By using the conventional technology, the micro-lens material layer (not shown) is first spin coated on the surface of the spacer layer 36. The thinnest possible thickness for the micro-lens material layer formed by using the presently available spin coating technology is between about 1.3 xcexcm and about 1.4 xcexcm. This is achieved at a spinning speed of about 4500 RPM. By using the conventional technology, the focal length of the micro-lens fabricated is about 5xcx9c6 xcexcm for a 6 xcexcm diameter micro-lens. The focal length achieved is not sufficient for the new generation color filters, i.e. for 0.35 xcexcm technology CMOS image sensors which require a focal length of at least 7 xcexcm and preferably 10 xcexcm. The longer focal length is necessary in order for a light beam to focus on the photodiode 24 that is formed in the substrate 26. A multiple number of micro-lenses is shown in a plane view of FIG. 3B while an enlarged, cross-sectional view of a single micro-lens formed by the conventional method with a large diameter is shown in FIG. 3B.
The conventional CMOS image sensor shown in FIG. 2B has only one metal layer 28 formed in a passivation layer 30 and a planarization layer 34. In modern semiconductor devices used as image sensors, a multiple number of metal layers, for instance 3 metal conductor layers are utilized in a 0.35 xcexcm technology CMOS image sensor. The small diameter of the micro-lens 12, which is contributed by the large thickness of the micro-lens material spin-coated (FIG. 2A) cannot produce a micro-lens that has the necessary focal length of at least 7 xcexcm and preferably at least 10 xcexcm. In order to reduce the thickness of the micro-lens material, as shown in FIG. 2A, a significantly higher rotational speed must be utilized in a spin coating process, i.e. a speed higher than 4500 RPM for producing a thickness of 1.4 xcexcm micro-lens material. At such high rotational speed, various other processing problems can be caused which include a possible loss of wafer from the wafer platform due to a break in vacuum used in holding the wafer. It is therefore impossible to vary the processing parameters of a spin coating process in order to provide a thinner layer of the micro-lens material, or to produce a multiplicity of micro-lenses each having a focal length longer than 7 xcexcm.
It is therefore an object of the present invention to produce a long focal length micro-lens in an image sensor application that does not have the drawbacks or shortcomings of conventional methods.
It is another object of the present invention to provide a method for making a long focal length micro-lens for use in color filters that have a focal length of at least 7 xcexcm.
It is a further object of the present invention to provide a method for making a long focal length micro-lens for colored filters that is suitable for fabricating 0.35 xcexcm technology CMOS image sensors.
It is another further object of the present invention to provide a method for making a long focal length micro-lens for colored filters that can be used for CMOS image sensors that have at least two metal conductor layers.
It is still another object of the present invention to provide a method for making a long focal length micro-lens for color filters that is suitable for producing a micro-lens material layer of less than 1 xcexcm thick.
It is yet another object of the present invention to provide a method for making a long focal length micro-lens for color filters suitable for use in CMOS image sensors wherein a micro-lens material is first coated to a thickness of less than 1 xcexcm, and then patterned and reflowed into micro-lenses having a focal length larger than 7 xcexcm for 6 xcexcm diameter micro-lenses.
It is still another further object of the present invention to provide a color filter that has a multiplicity of long focal length micro-lenses built thereon wherein each of the multiplicity of micro-lenses has a focal length of at least 10 xcexcm for 6 xcexcm diameter micro-lenses.
It is yet another further object of the present invention to provide a color filter that has a multiplicity of long focal length micro lenses built thereon for a CMOS image sensor application that contains at least two metal conductor layers with metal conductors embedded in at least two insulating material layers.
In accordance with the present invention, a method for making long focal length micro lenses for color filters and a device made by such method are provided.
In a preferred embodiment, a method for making long focal length micro-lenses for color filters can be carried out by the operating steps of first providing a semiconductor substrate that has at least one photodiode formed in a top surface; forming at least two metal conductor layers with metal conductors embedded in at least two insulating layers; forming a photoresist layer on top of the at least two metal conductor layers, the photoresist layer further includes a color agent therein for forming color filters of at least one of red, green and blue colors; depositing a layer of micro-lens material on top of the photoresist layer; patterning the layer of micro-lens material into at least four discrete regions for each micro-lens with pre-set spacing therein between; and reflowing the at least four discrete regions for the micro-lens and forming the micro-lens.
The method for making a long focal length micro-lens for color filters may further include the step of forming the at least two metal conductor layers to a thickness of at least 7 xcexcm, and preferably to a thickness of at least 10 xcexcm. The method may further include the step of forming the photoresist layer containing a color agent of a pigment or dye therein for forming color filters of red, green or blue. The method may further include the step of depositing a layer of micro-lens material of polymeric nature on top of the photoresist layer, or the step of depositing a layer of micro-lens material selected from the group consisting of acrylic and epoxy novolac resin, or the step of depositing a layer of micro-lens material that is transparent on top of the photoresist layer.
The method for making long focal length micro-lens for color filters may further include the step of patterning the layer of micro-lens material into at least four discrete regions in square shape for each micro-lens. The method may further include the step of patterning the layer of micro-lens material into at least four discrete regions in elongated strips for each micro-lens, or the step of patterning the layer of micro-lens material into at least nine discrete regions in square shape for each micro-lens, or the step of forming the pre-set spacing between the at least four discrete regions to larger than 0.01 xcexcm. The method may further include the step of providing the semiconductor substrate that has at least one photodiode formed in a top surface to a depth of at least 0.1 xcexcm.
The present invention is further directed to a color filter that has a multiplicity of long focal length micro-lenses build thereon including a semiconductor substrate that has at least one photodiode formed in a top surface; at least two metal conductor layers with metal conductors embedded in at least two insulating layers; a photoresist layer on top of the at least two metal conductor layers, the photoresist layer further includes a color agent therein for forming color filters of at least one of red, green or blue color; a multiplicity of micro-lenses on top of the photoresist layer each positioned corresponding to one of the color filters of at least one of red, green or blue color, the multiplicity of micro-lenses each has a focal length of not less than 7 xcexcm.
In the color filter that has a multiplicity of long focal length micro-lenses built thereon, the multiplicity of micro-lenses each having a focal length of preferably not less than 10 xcexcm. The at least two metal conductor layers may include three metal conductor layers with metal conductors embedded in three insulating material layers, the at least one photodiode formed in the top surface of the semiconductor substrate may have a thickness of at least 0.1 xcexcm. The at least two insulating material layers are formed of dielectric materials. The color agent may be a dye or a pigment. The color filters may be formed of red, green or blue color. The layer of micro-lens material is formed of a polymeric material, a transparent material, or an acrylic or epoxy novolac resin.